1. Field of the Invention
The present invention relates to fuel cells, and it particularly relates to a fuel cell which is operable under the unhumidified conditions.
2. Description of the Related Art
Solid polymer fuel cells in which a solid polymer membrane is used as the electrolyte are known as a type of fuel cells. The solid polymer fuel cells use a proton conductive polymer electrolyte membrane as the electrolyte and generally include an anode (fuel electrode), a cathode (oxidizing agent electrode) and a polymer electrolyte membrane disposed between the anode and the cathode. The anode of a solid polymer-fuel cell is provided with a catalyst layer that accelerates the oxidation of the fuel, and the cathode thereof is provided with a catalyst layer that accelerates the reduction of the oxidizing agent.
The fuel to be supplied to the anode of a solid polymer fuel cell is generally hydrogen, hydrogen-containing gas, mixed vapor of methanol and water, aqueous solution of methanol or the like. The oxidizing agent to be supplied to the cathode of a solid polymer fuel cell is generally oxygen, oxygen-containing gas or air.
The material generally used for the polymer electrolyte membrane is sulfonated highly-fluorinated polymer, which has the principal chain composed of fluorinated alkylene and the side chain composed of fluorinated vinyl ether having a sulfonic acid group at the end. This type of polymer electrolyte membrane, impregnated with the supply of an appropriate amount of water, displays sufficient ionic conductivity for generating power.
The conventional solid polymer fuel cells therefore required the moisture control of the polymer electrolyte membrane, which in turn led to the complexity or large size of the fuel cell systems.
In addressing such problems caused by the moisture control of the polymer electrolyte membrane, an unhumidified electrolyte membrane that ensures proton conductivity in unhumidified conditions has been developed as a substitution for the conventional polymer electrolyte membrane.
For example, Reference (1) in the following Related Art List discloses polybenzimidazole doped with phosphoric acid as a material for an unhumidified polymer electrolyte membrane.
Related Art List
(1) Japanese Patent Application Laid-Open No. Heill-503262.
Problems with known unhumidified polymer electrolyte membranes, however, have included the outflow of a strong acid, such as phosphoric acid, with a long time of operation, which tends to drop the open-circuit voltage and increase the cell resistance, thus impairing the stable operation of the fuel cell.